The present invention relates generally to circuit wiring boards and, more particularly, to ceramic composite circuit wiring boards and/or multichip modules and methods to construct the same.
Semiconductor integrated circuits (xe2x80x9cSICxe2x80x9d) or semiconductor chips are being developed to operate at increasingly higher speeds and to handle larger volumes of data. This trend has caused the density of electrical interconnections required between the semiconductor chip and the larger electronic system to increase dramatically. Conversely, this ultra-large scale integration restricts the physical dimensions of the SIC. The drive to implement more sophisticated SIC""s which require much larger numbers of electrical interconnections to be crammed into smaller physical dimensions creates a technical bottleneck, wherein SIC performance is increasingly limited by the circuit board/package connecting the chip to the larger electronic system.
The industry convention has been to use a lead frame that electrically interconnects the SIC to a printed circuit board (xe2x80x9cPCBxe2x80x9d), and to envelop the chip and lead frame in a ceramic laminate package. The packaged SIC is socketed to the PCB, which electrically connects the SIC to the larger electronic system. The modern, more sophisticated SIC""s generate greater amounts of heat than their predecessors. This heat, if not dissipated from the SIC, reduces circuit performance. Robust lead frames have been able to function as both electrical connection and heat sink, however, as the density of leads per unit area has increased, the physical dimension of the individual lead must be shrunk. Smaller lead sizes sharply limit their function as a heat sink. This has forced system manufacturers to dissipate thermal loads through unmanageably large heat sinks attached to the SICs, which hampers the drive towards smaller, mobile platforms.
Furthermore, the operating speed of the more sophisticated SICs is increasingly limited by the printed circuit board. Conventional PCBs have routed electrical signals between system and SIC through an electrode network patterned on the PCB surface on which the semiconductor chip is mounted. To allow the SIC to operate at higher speed the interconnections between the semiconductor chip and the electronic system must be low-resistance. Lower resistance electrical contact is achieved by shortening electrode length and by decreasing electrode resistivity. Shorter electrode lengths are engineered by embedding an electrical interconnection network within the circuit board rather than one patterned on the surface. The prior art discloses methods to construct multilayer ceramic composite printed circuit boards with electrical interconnection networks embedded within the circuit boards. However, these methods are performance-limited because the embedded electrode network is composed of metallic films, conducting pastes, or both, which have much higher electrical resistance than the wire form of the same conducting metal. Lower-resistance at higher signal frequency is also enhanced by forming the wiring board from low dielectric constant materials. Therefore, circuit wiring board and multichip module designs that comprise electrode networks of conducting metal wire embedded within a low dielectric ceramic, such as silica or alumina, and simultaneously contain heat sinks, embedded within the ceramic to dissipate heat generated by the SIC would be highly desirable.
Relevant prior art includes the following patents. Fujita et al., U.S. Pat. No. 5,396,034, discloses methods to construct a thin film ceramic multilayer wiring hybrid board. Bonham et al., U.S. Pat. No. 5,396,032, discloses the construction of a multi-chip module (xe2x80x9cMCMxe2x80x9d) with two sets of lead frames, one set supplying input/output bond pads, and another independent set to provide electrical contact to test pins that can be used to isolate and examine the performance of one or multiple devices mounted on a substrate within a cavity of said MCM, wherein the device(s) is (are) wire bonded to said pads. The material comprising the MCM package body can be ceramic, plastic, laminate, or metal, but the substrate on which the devices are mounted does not contain internal electrical interconnects and/or heat sinks. Wiesa, U.S. Pat. No. 5,375,039, discloses the construction of a printed circuit board with internal heat dissipation means channeling heat from power units mounted on the board to heat sinks, wherein the core of the printed circuit board comprises glass cloth. Chobot et al., U.S. Pat. No. 5,363,280, discloses methods to construct a multilayer ceramic circuit board in which some metal film layers function as electrode networks, and are separated from other metal film layers which function as heat sinks. Ohtaki et al., U.S. Pat. No. 5,300,163, discloses a process to fabricate a multilayer ceramic circuit board comprising a ceramic substrate, multiple layers of green tape with conductive paste patterns therein, and via holes with conductive paste to electrically interconnect the assembled layers. Cherukuri et al., U.S. Pat. No. 5,256,469, discloses a multilayered co-fired ceramic-on-metal circuit board prepared using ceramic green tapes and a system of low-temperature, high expansion glass ceramics. Capp et al., U.S. Pat. No. 5,113,315, discloses the construction of ceramic circuit board structures in which heat dissipation extensions are embedded in the ceramic member by laser drilling holes into the ceramic member and filling the holes with conductive metal using well-known metal deposition techniques. Plonski, U.S. Pat. No. 4,679,321, discloses a method of making interconnection boards with coaxial wire interconnects on the external major surface of the board substrate that opposes the major surface upon which integrated circuits are mounted. Ushifusa et al., U.S. Pat. No. 4,598,167, discloses the construction of multilayered ceramic circuit board that comprises a plurality of integrally bonded ceramic layers, each having a patterned electrically conducting paste layer and through holes filled with electrical conductors for connecting the patterned electrically conducting layers on respective ceramic layers to form a predetermined wiring circuit. Takeuchi, U.S. Pat. No. 4,551,357, discloses a manufacturing process for ceramic circuit boards that comprises firing a circuit pattern formed from an organic-laden conductive paste on the surface of a green-state ceramic with an organic binder.
It is therefore an object of the present invention to provide a composite wiring structure which enhances SIC performance.
It is another object of this invention to provide a composite circuit wiring structure which increases the allowable operating speeds of SICs.
It is a further object of this invention to reduce compressive and shear stresses within the composite structure.
It is another further object of this invention to provide a composite circuit wiring board structure wherein the structure""s dielectric member is either a ceramic or an organo-ceramic composite.
It is still another object of this invention to provide a highly efficient and effective ceramic composite wiring structure for SICs and the method of manufacture thereof.
The objects set forth above as well as further objects and advantages of the present invention are achieved by the preferred embodiments of the invention described herein.
The preferred embodiments include a composite circuit wiring structure having one or more electrodes on one major surface of a dielectric member, and wherein a semiconductor integrated circuit (xe2x80x9cSICxe2x80x9d) is placed in direct electrical contact with the electrodes which are electrically contacted, through an electrical interconnection network within the dielectric ceramic member, to an external input/output signal driver.
Still further the preferred embodiments provide a composite circuit wiring structure wherein the dielectric member also contains an embedded thermal distribution network.
Even further the preferred embodiments provide a composite circuit wiring structure having one or more electrodes on one major surface of a dielectric member, and wherein at least one SIC is placed on a mounting area and electrically contacted to at least one electrode through a conducting wire means.
Still further the preferred embodiments reduce thermally generated compressive or shear stresses between the circuit wiring board""s dielectric member and the embedded electrical interconnection network or the embedded thermal distribution network through the use of networks with curved joints.
Even further this invention reduces thermally generated compressive or shear stresses between the circuit wiring board""s dielectric member and the embedded electrical interconnection network or the embedded thermal distribution network through the application of organic resins with high thermal decomposition temperatures to the networks prior to embedding the networks in the ceramic member.
Still further the present invention permits the inclusion of blocking capacitors with the dielectric member of the composite wiring structure.
Even further still the present invention provides the above-mentioned embodiments to be constructed with a ceramic or an organo-ceramic material as the dielectric member of the composite wiring structure.
More specifically a preferred embodiment of this invention relates to a dielectric (ceramic or organo-ceramic) composite circuit wiring board having one or more electrodes on one major surface of a ceramic member and wherein a semiconductor integrated circuit (xe2x80x9cSICxe2x80x9d) is placed in direct electrical contact with the electrodes. The SIC is electrically contacted, through an electrical interconnection network, made up of a conductive wire, preferably of copper wire, to other SIC""s electrically contacted to other electrodes on the circuit wiring board""s major surface, and/or to an external input/output signal driver that is electrically contacted to the ceramic circuit wiring board. This is accomplished either through yet another electrode on the circuit wiring board""s major surface, or through a segment of conductive wire, connected to the electrical interconnection network, that protrudes through a minor surface of the circuit wiring board""s dielectric member. The dielectric member also contains an embedded thermal distribution network of heat sinks, formed from elongated thermally conducting material such as metal, or hollow tubes in which a heat absorbing fluid is circulated. The embedded thermal distribution network is located in the vicinity of, but not in direct contact with, the electrodes making direct electrical contact with the SIC, and the terminal points of the embedded heat sinks protrude through a minor surface of the ceramic member to make thermal contact with a further heat sink or thermal reservoir that is external to the circuit wiring board.
As an example, the dielectric member comprises aluminate or silicate ceramic phases. Silica ceramic phases are particularly preferred to reduce the level of signal attenuation through dielectric loss mechanisms at higher signal frequencies. Another metal member is bonded to the major surface of the circuit wiring board""s ceramic member that opposes the major surface on which the SIC is contacted to the electrodes. The invention also encompasses methods to construct the circuit wiring board structure through low-temperature processing methods.
The use of solution precursors allows ceramic to be formed around the network assemblies by filling the area bordered by the mold materials, mounting supports, and the base metal member with liquid precursor and driving the chemical reaction that transforms the liquid precursor into the corresponding solid state ceramic. The invention preferably incorporates therein the use of metalorganic precursors, whereby metal precursors to the ceramic oxide are first reacted with a carboxylic acid, such as 2-ethylhexanoic acid, to form a solution of carboxylic acid salts in organic acid solution. However, other solution processing techniques, such as sol-gel techniques, could also work as effectively and is considered to be within the spirit and scope of the invention.
The area filled with liquid precursor is filled with ceramic after the transforming chemical reaction is completed. As described below, the transforming chemical reaction bonds the ceramic to the network assemblies, the metal member, and the walls of the bordering mold materials and/or mounting supports. The liquid properties of the solution allow precursor materials to uniformly envelop the network assemblies. When metalorganic precursors are used, pyrolytic action decomposes the carboxylic acid salts into their corresponding metal oxides. Unstable metal oxide radicals are formed as a result of pyrolysis, which rapidly bond to stable organic and inorganic surfaces that are part of the network assemblies, the base metal member, and the mold materials and/or mounting supports. The unstable metal oxide radicals also bond with other decomposing metal oxide radicals to form a contiguous ceramic network.
The volume fraction of solid state ceramic is less than the volume solution precursor as the decomposition (or unwanted reaction) products are removed. Thus, it is advantageous to utilize a high solid content precursor solution, which is often quite viscous; or to pyrolyze the precursor in situ as it is applied, as is the case when the precursors are spray pyrolyzed onto an already heated assembly.
The action of spray pyrolysis allows undesirable reaction by-products, such as the precursor solvent and decomposition products to be physically removed at a much faster rate than the ceramic precursors are applied and simultaneously formed into ceramic. Thus, spray pyrolysis allows a higher volume fraction of solid state ceramic to occupy the region into which it is being applied.
The present invention also permits the formation of an organo-ceramic dielectric, if such a dielectric member is desired, through the incomplete decomposition of the dissolved metalorganic ceramic precursors. The present invention forms metalorganic precursors by directly or indirectly reacting the metal precursor(s) with a carboxylic acid solvent to produce a solution of carboxylic acid salt(s) dissolved within the carboxylic acid. 2-Ethylhexanoic acid is a preferred solvent and has a flash point of 210 degrees C. 2-Ethylhexanoate precursor salts will typically begin to decompose over temperatures in the range of 225-375 degrees C., depending upon the chemistry of salt""s metal. Thermal decomposition is usually complete at temperatures above 400-475 degrees C. A composite organo-ceramic dielectric can be formed by spray-pyrolyzing the solution on to the circuit wiring board assembly heated to temperatures above the initial decomposition temperature(s) of the dissolved carboxylic acid salt(s), (225-375 degrees C.), yet below the temperatures at which the salt(s)""s organic ligands thoroughly decompose, (400-475 degrees C.). During spray-pyrolytic decomposition the carboxylic acid evaporates, depositing waxy carboxylic acid salts that decompose in situ. When the circuit wiring board assembly is heated to an appropriate temperature, the resultant dielectric material is a matrix of fully deflagrated oxide ceramic with incompletely decomposed organic material, thereby producing an organo-ceramic dielectric member.
Another embodiment of the present invention relates to a dielectric (for example, the dielectric being either a xe2x80x9cpurexe2x80x9d ceramic or organo-ceramic) composite circuit wiring board that comprises a metal member including one or more electrodes, and one or more mounting areas, all on one major surface of a ceramic member. At least one SIC is placed on the mounting area and electrically contacted to at least one electrode through a conducting wire means. The SIC is in further electrical contact, through an electrical interconnection network to other SIC""s in electrical contact with other electrodes on the dielectric circuit wiring board""s major surface, or to an external input/output signal driver that is electrically contacted to the dielectric circuit wiring board either through yet another electrode on the circuit wiring board""s major surface, or through a segment of conductive wire, preferably copper wire, connected to the electrical interconnection network, that protrudes through a minor surface of the circuit wiring board""s dielectric member. The dielectric member also contains an embedded thermal distribution network which includes heat sinks formed from elongated thermally conducting material such as metal, or hollow tubes in which a heat absorbing fluid is circulated. The thermal distribution network may or may not be in thermal contact with the SIC through the mounting area, and the terminal points of the embedded heat sinks protrude through a minor surface of the dielectric member to make thermal contact with a thermal reservoir that is external to the circuit wiring board. The dielectric member may be composed of aluminate or silicate ceramic phases. Another metal member is bonded to the major surface of the circuit wiring board""s dielectric member that opposes the major surface on which the SIC is contacted to the mounting area and the electrodes.
Two methods are employed within the present invention to reduce the deleterious effects of stress on the dielectric member and the embedded network structures. The first deploys curves in the design of the embedded network structures to high stress points that result from sharp edged structures. When the network structures are designed with curved, rather than sharp-cornered L-joints and T-joints the stress is more evenly distributed over the radial arcs, as opposed to building up intense compressive forces at the sharp critical points in the network. Second, compressive stress is also reduced in the network by coating the (copper) metal wire forming the electrical interconnection network and the heat pipes forming the thermal dissipation network with an organic resin.
For a better understanding of the present invention, together with other and further objects thereof, reference is made to the following description taken in conjunction with the accompanying drawings and its scope will be pointed out in the appended claims.